The term, pressure sensitive adhesive, as used herein refers to adhesives that can be adhered to a surface and yet can be stripped from said surface without transferring more than trace quantities of adhesive to the surface, and can be readhered to the same or another surface because the adhesive retains some or all of its tack and adhesive strength.
Pressure sensitive adhesives, hereinafter also referred to as PSAs, comprising polyorganosiloxanes are not new. Mixtures of a benzene soluble resin copolymer comprising EQU R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units,
hereinafter also referred to as MQ resin, M units, and Q units respectively, and a vinyl endblocked polydiorganosiloxane have been disclosed, which have PSA characteristics. U.S. Pat. Nos. 3,983,298 and 4,774,297 teach such mixtures. U.S. Pat. No. 3,983,298 points out that compositions of the prior art based on mixtures of MQ resins and low viscosity silicones do not form pressure sensitive adhesive compositions and then teaches that the use of mixtures of such benzene soluble resins with higher molecular weight linear vinyl functional polysiloxane having viscosities of 20-100,000 centipoises does form PSA's. U.S. Pat. No. 4,774,297 also teaches the preparation of PSA's utilizing vinyl functional polysiloxanes of still higher molecular weight with organopolysiloxane resins. In order to obtain satisfactory products, U.S. Pat. No. 4,774,297 teaches that it is essential that the vinyl functional polysiloxane have a viscosity of at least 500,000 centipoise and preferably at least 1,000,000 centipoise. The compositions of U.S. Pat. Nos. 3,983,298 and 4,774,297 both require solvent for the purpose of reducing viscosity of the PSA to a usable level. In particular, U.S. Pat. No. 4,774,297 requires about 25 to 400 parts of solvent per 100 parts of PSA composition. U.S. Pat. No. 3,983,298 requires less solvent, but when 95 parts of composition are blended with 5 parts of solvent the viscosity remains 162,000 centipoise at 25.degree. C. This is rather greater than is usually desired. At 90-95% solids a viscosity of less than 100,000 and preferably less than 30,000 is desirable.
The PSA art still requires a polyorganosiloxane PSA having high tack, good adhesive strength, and low solvent content. In light of the foregoing it was unexpected to obtain these desirable qualities by using low viscosity vinyl-functional polysiloxanes in critical combinations and curing the resulting mixture.